Method of making a pocket-type filter and product

ABSTRACT

An aqueous slurry is prepared which contains no substantial amount of phenol thermoset resin but has up to 30 percent wood pulp; 20-65 percent by weight of either olefin, acrylic, modacrylic, or vinylchloride-vinyl acetate copolymer fibers; and a balance of either rayon, polyester, polyamide, fiberglass, asbestos or a mixture of any two or more. Flame retardants may also be added. The slurry is suction molded to produce a pocketshaped filter for removing contaminants from streams of gas.

United States Patent Inventor Paul D. Burnham Watervllle, Maine Appl.No. 889,091 Filed Dec. 30, 1969 Patented Nov. 23, 1971 Assignee KeyesFibre Company Waterville, Maine Continuation-in-part of application Ser.No. 701,895, Jan. 31, 1968, now abandoned. This application Dec. 30,1969, Ser. No. 889,091

METHOD OF MAKING A POCKET-TYPE FILTER AND PRODUCT 16 Claims, 8 DrawingFigs.

U.S. Cl 162/146, 55/418, 55/482, 55/524, 55/528, 156/62.2,

161/150, 162/218, 162/227, 210/505 1nt.Cl D2lh 5/12 F leld 01' Search162/ 145, 146, 218, 227, 228, 231; 55/418, 482, 524, 528; 210/505, 508;161/150; 156/622 [56] References Cited UNITED STATES PATENTS 2,802,4058/1957 Krogel 162/231 X 2,928,765 3/1960 Kurjan 55/528 X 2,933,1544/1960 Lauterbach 55/528 X 3,028,911 4/1962 De Lear 162/227 X 3,158,53211/1964 Palletal. 162/145 X 3,306,815 2/1967 Mayne 162/228 X 3,386,2316/1968 Nutting 162/146 X 3,442,757 5/1969 Williams 162/227 X PrimaryExaminer-S. Leon Bashore Assistant ExaminerRichard H. AndersonAn0rney-Connolly and Hutz ABSTRACT: An aqueous slurry is prepared whichcontains no substantial amount of phenol thermoset resin but has up to30 percent wood pulp; 20-65 percent by weight of either olefin, acrylic,modacrylic, or vinylchloride-vinyl acetate copolymer fibers; and abalance of either rayon, polyester, polyamide, fiberglass, asbestos or amixture of any two or more. Flame retardants may also be added. Theslurry is suction molded to produce a pocket-shaped filter for removingcontaminants from streams of gas.

PATENTEDunv 23 I91:

SEEET 1 BF 2 METHOD OF MAKING A POCKET-TYPE FILTER AND PRODUCTCROSS-REFERENCE TO RELATED APPLICATIONS This is a continuation-in-partof US. application Ser. No. 701,895 filed Jan. 3 l 1968, now abandoned.

BACKGROUND OF THE INVENTION The claimed invention relates to the fieldof articles molded by the suction deposition of fibrous pulp materialsfrom an aqueous slurry against screen-covered, open-face forming dies,and particularly to improved techniques for molding components for theassembly of large disposable bagor pocket-type filters useful forremoving contaminants from fluid streams.

Prior to the present invention, large pocket or bag type filter units ofvarious shapes and constructions have been known, such as the onesillustrated in Rivers US. Pat. No. 2,853,154, Hagendoom US. Pat. No.3,099,547 and Hogg US. Pat. No. 3,124,440. Such filters have heretoforebeen formed from flat sheets of flexible filtering material which isformed to the desired contoured configuration by pleating, creasing,folding and other shaping techniques. The shaping of the sections whichgo to make up the filter unit is an expensive and time consumingoperation which must be figured as an addition to the initial cost ofthe flexible sheet material. Since the filter units of this nature arepreferably disposable, the initial cost factor can spell the differencebetween commercial success and failure.

In the past, it has been proposed to directly mold filtering units fromfibrous material, as in Anderson US. Pat. No. 2,539,767. This procedureinvolves the well-known suction deposition of such pulp material from anaqueous slurry against an open-face forming die. The fibrous materialaccumulates on the face of the die in matted, interfelted fashion which,upon removal from the die and drying, constitutes a continuous sheetdirectly molded to virtually any desired shape. Such molded fibrousunits, however, have certain drawbacks in that the strength andflammability of such material has been considered inadequate for largebagor pocket-type filters of the type referred to above. If the fibrouspulp material chosen is of the type and density which is known toproduce stronger molded articles, perhaps with the addition of resins orother strengthening binders, the porosity of the resultant moldedproduct is insufficient to provide proper air flow of the requisitecommercial efficiency. Binder resins such as phenol thermoset typeresins in amounts of 15 to 25 percent on the weight of the fibers, assuggested in Nutting US. Pat. No. 3,386,231, have recently beenexperimented with in molding large bagor pocket-type filters of the typein question, but they have proved unsatisfactory because they areextremely difficult to render flame resistant and they have a propensityto deposit on the fibers in large chunks which adversely effects theairflow characteristics of the finished product. If, on the other hand,extremely large bagor pockettype filters are directly molded by thesuction process from materials and with density characteristics adequateto provide for the required airflow, the resultant structure heretoforehas had insufficient strength and flame resistance to be acceptable forcommercial use.

In attempting to solve the longstanding problem and provide a largebagor pocket-type filter directly molded by the suction process to thedesired shape, conventionally moldable fibrous materials have provedinappropriate. Speculations concerning different natural and syntheticmaterials which might be useful for nonwoven filtering units areavailable, such as in Lauterbach US. Pat. No. 2,933,154 and Pall US.Pat. No. 3,158,532, but many of such suggested materials are whollyinappropriate to molding by the suction deposition process. Such randomsuggestions of numerous materials thus amount to no more than aninvitation to experiment with the materials but provide no useful clueconcerning the proper combinations of materials for the successfulmolding of large bagor pocket-type filters by the aqueous slurry suctionprocess.

The problems encountered in providing the desired disposable filterinvolve two critical balances in addition to low cost. First, thefiltering characteristics of the material must be balanced against thestrength of the material, as noted above. Second, the filteringcharacteristics of the unit involve a critical balance between the airflow capacity on the one hand and the contaminant collecting capacity onthe other hand.

Thus, the long-recognized problem which has remained unsolved until theadvent of the present invention is the provision of a large bagorpocket-type filter with the heretofore irreconcilable properties ofrequisite strength and desirable filtering characteristics directlymolded from a novel combination of materials by the suction depositionprocess into serviceable, disposable filtering units.

SUMMARY OF THE INVENTION In accordance with the present invention, thereis provided a suction molded filter unit for filtering contaminants fromstreams of gas, and a method for manufacturing the unit, which involvespreparing an extremely thin aqueous slurry preferably having less than0.4 percent by weight of primarily synthetic fibrous material. Thefibrous material includes no more than about 30.0 percent by weight ofwood pulp, a large amount of at least one member of the group consistingof olefin, acrylic and modacrylic fibers and fibers of a copolymer ofvinyl chloride and vinyl acetate, and a balance comprising fibers ofrayon, a polyester, :1 polyamide, fiberglass or asbestos or a mixturethereof. A layer of the fibrous material is formed by suction against anopen-face forming die contoured to define the shape of the filter unit,and the layer of fibrous material is transferred from the forming die tothe dryer conveyor by means of a matingly contoured transfer die. Theforming die comprises a slotted die construction with a screen covering,whereas the transfer die comprises a slotted die construction without ascreen covering.

BRIEF DESCRIPTION OF THE DRAWINGS Numerous advantages of the presentinvention will become apparent to one skilled in the art from a readingof the detailed description in conjunction with the accompanyingdrawings wherein similar reference characters refer to similar parts,and in which:

FIG. 1 is a front or upstream elevational view of an assembled filterunit according to this invention;

FIG. 2 is a side elevational view of the filter unit of FIG. 1;

FIG. 3 is a rear or downstream elevational view of the filter unit ofFIG. 1;

FIG. 4 is a fragmentary enlarged sectional elevational view on line 4-4of FIG. 2;

FIG. 5 is a plan view of a single molded unit from line 5-5 of FIG. 2;

FIG. 6 is a diagrammatic illustration of a machine on which the filterunit of FIG. 1 may be molded;

FIG. 7 is a plan view of a single molded unit which differs from that ofFIGS. 1-5; and

FIG. 8 is an enlarged exploded perspective view of several of the unitsof FIG. 7 assembled together to provide a filter unit which differs fromthat of FIGS. 1-3.

DETAILED DESCRIPTION OF THE INVENTION Referring in more particularity tothe drawing, there is shown in FIGS. 1-3 a replaceable unit 10 forfiltering contaminants from streams of gas. The unit comprises a seriesof pocket forming layers 12 superimposed one upon another in layeredfashion. The replaceable unit conveniently has a width dimension and aheight dimension which are approximately identical, whereby the frontand rear ends of the unit are square. The length and other dimensions ofthe unit may vary in wide ranges depending upon the filteringenvironment in which it is to operate.

The front end 14 may conveniently be referred to as the upstream end ofthe unit, while the rear end 16 may conveniently be referred to as thedownstream end of the unit. The layered pocket forming sections areattached together and to a framework 18 at the upstream end of the unit,and they retain their properly assembled relation at the downstream endof the unit without additional attaching means. The framework 18conveniently may comprise a front member 20 and a rear member 22, bothbeing of open rectangular configuration. Marginal portions of theupstream end of the filtering units are folded outwardly and clampedbetween the frame members 20, 22 and the frame members secured to thefiltering layers in this manner by contact cement or other suitableadhering means. When the replaceable unit is assembled in this fashion,it may be installed in suitable existing openings connected with the airstream from which contaminants are to be filtered. AS is known, when onefiltering unit has completed its life service, it may simply be removedand discarded and a new replaceable unit 10 substituted in the airstream.

In addition to securing the various pocket forming layers 12 together bymeans of the framework 18, each of the various layers may be secured toits adjacent layer as at 24. This may conveniently be accomplished byproviding protruding lip or flange portions 25 which extend laterallyacross the entire upstream face of each layer. When the layers areassembled to complete the final product 10, the flanges 25 of adjacentlayers may be secured together by heat sealing or other appropriateadhering means. Such heat sealing with the combination of materialsdiscussed below is a function of heat, pressure and time well known tothose skilled in the art. In a suggested replaceable unit 10, there maybe of the layers 12, each layer having a series of pockets formedtherein.

Each layer 12 is comprised of two subassembly halves 26, heat sealedtogether to provide the series of elongated pockets 28 which are open atthe upstream end 14 of the final unit and closed at the downstream end16 of the final unit. As is well understood in the filtering arts,- theair stream in which the filtering unit is placed enters the pockets 28at the open end and is forced through the filtering material along thefull length of the pockets. In the assembled unit, the spacing 30between the pockets provides a channel into which the filtered air mayflow and freely escape from the downstream end 16 of the unit. Sinceeach half 26 of each layer 12 is identical, a detailed description ofonly one such half is necessary.

Each subassembly half 26 thus includes an upstream side 32 and adownstream side 34. Viewing one of the subassemblies 26 from theupstream side, as in FIG. 5, the unit has along its forward edge theaforementioned lip or flange 25. In approximately the same plane as thelip 25, but offset slightly therefrom, is a secondary flange 36 whichsurrounds the remaining three sides of the subassembly. When two suchsubassemblies are positioned with their upstream sides 32 face to face,the surrounding flanges 36 make planar contact so that the twosubassemblies may be secured together by heat sealing or other means toform one of the pocket layers 12. Each subassembly 26 further includes aseries of elongated depressions or valleys 38 which are open at theupstream end and closed at the downstream end. When two subassembliesare secured together to form a layer 12, the opposed valleys 38 form theaforementioned elongated pockets 28. The adjacent valleys 38 in eachsubassembly 26 are separated by parallel ribs 40 having elongated flattops which are coplanar with the surrounding flange 36. As can beappreciated, when two subasse'mblies are secured together, the rib tops40 may be sealed together along with the surrounding flanges 36 toinsure that the individual pockets are isolated from each other.

Referring to FIGS. 7 and 8, a subassembly 26' is disclosed which hascertain advantages over that described immediately above. Thesubassembly half 26' in FIG. 7 differs in minor but important respectsfrom the subassembly half 26 illustrated in FIG. 5. In the FIG. 5embodiment, the mouth portion of the filter pockets have a commonintegral supporting portion member of the same material as the sheets,surrounding and 5 jacent pockets are unnecessary for supportingpurposes, and

they also create a buffeting effect which reduces the effectiveness offluid flow into pockets. This problem is corrected by the design in FIG.7 where the parallel ribs 40', which separate adjacent pocket-formingvalleys 38', are streamlined and tapered as at 41 to merge smoothly intothe flat plane of the forward edge lip or flange 25'. The outennostextent of the merging rib ends 41 are spaced the width of the flange 25from the edge of the sheet. In addition, when the subassembly halves 26of FIG. 5 are assembled into units, the flanges 25 which are joined infaced relation define longitudinally extending strengthening rib meansbetween rows of pockets, as shown in FIGS. 1 and 2, defining inlet fluidguide channels communicating with the mouths of the rows ofside-by-side' filter pockets. This feature is also corrected with thesubassembly half 26' of FIG.7 when assembled with an identicalsubassembly half as illustrated in FIG. 8. In assembly, the flanges 25'are folded back to lie in a downstream direction-rather than theiras-molded" upstream direction-and joined in back-to-back faced relation.This eliminates the rib means which otherwise would extendlongitudinally beyond the upstream extremity of the streamlined portion41 of the ribs 40, eliminating the possibility that a longitudinallyextending rib could bend sideways and partially obstruct an entire rowof pocket mouths.

Each subassembly 26 may be directly molded to shape against open-facesuction dies. The dies may be incorporated on conventional pulp moldingmachinery, such as that illustrated in FIG. 6 and in the left-handportion of the drawing in Randall US. Pat. No. 2,704,493. Such machineryis of the intermittent rotating type in which foraminous female formingdies 42, mounted on hollow radial arms 44, are rotated step by step by ashaft 46 through a pulp tank 48 for the suction formation of moldedarticles on the dies. At each pause of the rotation of the shaft 46, aformed article is removed from a forming die by means of a male transferdie 50 mounted on a hollow arm 52 carried by a shaft 54. The open-faceforming dies 42 and the transfer die 50 are provided with conventionalair and vacuum connections for forming and transferring the articles.

The shaft 54 is operated from an eccentric 56 fastened on a continuouslyrotating drive shaft 58, whereby the shaft 54 and its transfer die 50are reciprocated toward and away from the forming dies 42 in time to thestep by step rotation of the dies. As each forming die is paused inregistration with the transfer die, the transfer die moves toward theforming die to accomplish transfer of the formed article to the transferdie, after which the transfer die is swung laterally, by any suitablecam or the like (not shown) to discharge the formed article onto aconveyor 60 which carries it to another conveyor 62 operating within anopen drier 64 of conventional construction. The formed articles may besupported in any suitable manner on the conveyor 62, preferably byresting on the flat surface without constricting drying dies or otherarrangements.

The forming dies 42 useful according to this invention preferablycomprise what is commonly known in the molded pulp art as ascreen-covered slotted die. Such die construction is described in detailin Chaplin US Pat. No. 2,829,568. In the die according to thisinvention, however, approximately percent of the exposed forming areaconsists of screen supporting lands between the slots, and the slotsextend in parallel fashion across the open forming face of the diepreferably parallel with the longer sides of the rectangular formingarea of the die. Suction is applied through rearward openings connectingwith the base of the slots, and the suction is distributed by the screenover the entire forming face of the die. The large percentage of screenwhich is exposed to the slots enables a large volume of fluid to passthrough the die to enable the combination of materials which comprisethe pulp slurry,

discussed in more detail below, to deposit a suitable layer of fibrousmaterial on the screen face of the die.

The transfer die 50 may comprise what is known in the molded pulp art asa slotted die, that is one without a screen covering. Such a die isillustrated in Manson US. Pat. No. 2,273,055. This comprises a slottedmember wherein the lands between the slots make direct contact with thelayer of fibrous material which is transferred to this die from theforming die. Suction is applied through rearward openings connectingwith the base of the slots, and the die has approximately 95 percent ofits exposed article contacting area composed of lands between the slots.The transfer die useful in connection with this invention need not beequipped with heating means, as the pressing and transfer is donerapidly at room temperature, as discussed below.

The pulp slurry according to this invention comprises an aqueous slurrycontaining up to 0.4 percent by weight of an intimate admixture orsolids content, and preferably the fibrous material amounts to onlyapproximately 0.05 percent of the weight of the slurry. The slurry isprepared by first blending approximately 2 percent by weight of thefibrous material with approximately 98 percent of water, and thereafteradding water to the blend to acquire the desired aqueous slurrycontaining less than 0.4 percent by weight of fibrous material. Inpreparing the blend, the wood pulp discussed below is preferably firstmixed with the water, the hydrophobic fibers discussed below are nextblended in, and the ingredients such as rayon which blend in easily areadded last. Thereafter, before the additional water is added to attainthe desired density, a flame retarder may be added depending upon theultimate use of the final filter product, as well as approximately 1percent by weight cationic starch to improve the bonding of the fibrousmaterial and 1.5 percent alum to reduce the pH to insure that the flameretarder acts properly on the fibrous material. The flame retarderpresently found satisfactoryand which has useful bonding properties aswell-may be added to the slurry in amounts up to 50 percent or more onthe weight of the fibrous material after the rayon and before thestarch, and this advisably comprises, in approximately equal parts, athermoplastic (not thermosetting) resinous copolymer of vinylidenechloride, and antimony trioxide pigment in an approximately 5050 watersuspension, such as Vi-Tard" available from National Starch ChemicalCorporation.

The combination of ingredients may vary within the parameters outlinedbelow depending upon the particular filtering duties for which theproduct is intended. The preferred combination of ingredients for thefibrous material includes up to 30.0 percent by weight of wood pulp, andin one example preferably about 20 percent by weight. A suitable woodpulp is bleached softwood ltraft, a conventional papermaking ingredienthaving adequate fiber length, although equivalent fibers havingrecognized fibrillation qualities to promote interfelting, such asasbestos, may be substituted for the wood pulp if desired. The wood pulpis a very inexpensive ingredient and is easily wettable whichfacilitates moldability. Moreover, the wood pulp because of the smallfibers improves the dust catching ability for the fines or smallparticles, but large amounts of wood pulp over 30 percent by weightcorrespondingly reduce the air flow capacity of the final filter.

The preferred fibrous pulp material also includes 20-65 percent byweight of olefin, acrylic and modacrylic fibers covering a wide range,as well as fibers of a copolymer of vinyl chloride and vinyl acetate. Inone example preferably about 30 percent by weight of a copolymer ofvinyl chloride and vinyl acetate is used. The latter generally containsat least about 80 percent by weight of copolymerized vinyl chloride.Although all such copolymers are suitable for the purpose of the presentinvention, it is generally preferred to employ a copolymer containing atleast 85 percent by weight of copolymerized vinyl chloride. Vinyon" is atypical commercially available vinyl chloride-vinyl acetate copolymerwhich has proved to be quite satisfactory for use in the presentinvention. This copolymer is understood to contain 88-90 percent byweight of copolymerized vinyl chloride. This material is obtainablecommercially in fibrous form having a fiber length of approximatelyone-fourth a diameter of less than about microns, and in one exampleapproximately 15-20 microns. This material is fireproof and has thevaluable property of being heat scalable so that the subassemblies 26may be secured together to form the layers 12 of which the unit 10 iscomprised. Less than about 20 percent of this material thus results in aproduct which has inferior heat sealing capabilities and requiresadditional measures to achieve flameproof qualities. While more than 65percent of this material may be utilized, more than 20 percent-such as30 percent-of wood pulp should be utilized in such cases to insuremoldability.

The mixture of fibrous material also includes a balance comprisingfibers of rayon, a polyester such as polyethylene teraphthalate, apolyamide, fiberglass or asbestos or a mixture thereof. Preferably thismay include 10-40 percent by weight of fibers of rayon and 10-40 percentby weight of fibers of polyethylene teraphthalate. Most specifically, asone example suitable filters have been made which include about 30percent by weight of fibers of rayon having a length of up toapproximately three-eighths inch wherein a minor portion of the fibershave a diameter of approximately 15-20 microns and a major portion havea diameter of approximately 27-32 microns, and about 20 percent byweight of fibers of polyethylene teraphthalate wherein the fibers have alength of approximately three-eighths inch and a diameter ofapproximately 15-20 microns, although diameters of up to 100 microns arecontemplated. The rayon is preferred because it is one of the lower costmaterials which is commercially available with good diameterselectability for controlling the porosity of the final product. Butsince rayon is flammable, large amounts thereof require additional flameretardent. The polyethylene teraphthalate has excellent dust-holdingabilities in the final product, but is expensive so that large amountsof this ingredient are to be avoided except for filters where thecontaminant-catching requirements are sufficiently high to justify thecost of this ingredient.

While the above-specified ingredients,'and the particular ratiosthereof, are suitable for general filtering purposes, the followingcombination of ingredients has particular utility where fire is ahazard. To produce a filter having high resistance to flame, and theimportant ability to burn with a minimum amount of dark smoke, anaqueous slurry is first prepared containing from about 0.05 to about0.10 percent by weight of fibrous material. The fibrous materialincludes about 10 percent by weight of wood pulp, about 35 percent byweight of fibers of a copolymer of vinyl chloride and vinyl acetate,about 40 percent by weight of fibers of fiberglass, and about 15 percentby weight of fibers of polyethylene teraphthalate. To this combinationof ingredients, there is next added approximately 1.0 percent by weighton the fibrous material of finely divided asbestos particles to act as acharge to attract the flame retardant added next. About 30 percent byweight on the fibrous material of a thermoplastic resinous copolymer ofvinylidene chloride, and antimony trioxide pigment in an approximately50-50 water suspension is included as a flame retardant. Approximatelyl-ly percent by weight on the fibrous material of cationic starch toimprove the bonding of the fibrous material, 2.0 percent by weight onthe fibrous material of alum to reduce the pH to insure that the flameretardant acts properly on the fibrous material, and approximately Wpercent by weight on the fibrous material of dry powder soda ash toreduce acidity and aid in retaining the ammonium polyphosphate-addednext-on the fibrous material, are then added. Finally, approximately 20percent by weight on the fibrous material of what is believed to be anammonium polyphosphate, such as Phos-Chek" P/30 available from MonsantoCompany is added. Comparing the fire resistant combination ofingredients disclosed immediately above with the combination ofingredients for general purposes disclosed earlier, it will be notedthat the substitution of fiberglass for rayon improves flame resistance,and the addition of the Phos-Chek" P/30 is believed to reduce smoking,probably inhibits the spread of flame, and possibly minimizes theblackness of color in whatever smoke there is.

Successful filters have been manufactured according to this invention inthe following manner. First, the pulp slurry including the fibrous pulpmaterials of the types and in the quantities described above is preparedaccording to the procedure outlined above. This pulp slurry is deliveredto the pond 48 and contained there in the conventional manner. The abovedescribed screen-covered slotted forming die 42 is immersed in theaqueous slurry and suction or vacuum at a value of from about to inchesHg is applied for approximately one to ten seconds. Due to the unusuallylarge (20 percent) percentage of the slotted die face which is exposedto the suction, large amounts of fluid are drawn through the screenresulting the deposition of fibrous material upon the screen. Forinstance, for a subassembly half 26 of generally rectangular overallshape approximately 22Xl2 inches, approximately 25 gallons of fluid passthrough the die 42 at 20 inches Hg suction in a 6-second forming time.

The forming die with the layer of fibrous material thereon is thenremoved from the slurry and moved into position for engagement with thetransfer die 50. The layer of fibrous material on the face of theforming die at this stage is composed of approximately one part fibrousmaterial to four parts water. Thereafter, the matingly contouredtransfer die 50 is brought into contact with the layer of fibrousmaterial on the forming die, and lightly pressed thereagainst at roomtemperature for a short time such as several seconds or less. Suction orvacuum at a value of approximately 5 to 20 inches Hg is then applied tothe transfer die, and the suction applied to the forming die is shut offand air pressure applied in the known fashion to effect transfer of thelayer of molded fibrous material from the forming die to the transferdie. The transfer die thereafter moves into position above the dryerconveyor 60 and transfers the layer of molded fibrous material byreplacing suction with pressure in the known'manner to effect release ofthe layer from the transfer die to the dryer conveyor.

The filter unit subassembly 26 is then conveyed through the dryingtunnel 64 in the known fashion. Preferably, for one example with about50 percent by weight of wettable fibers, the drying takes place for 5-20minutes at an oven temperature of approximately 200-300 F. During thedrying process, the article attains a drying temperature of about240-250 F. which is in the softening range but below the melting pointof the copolymer of vinyl chloride and vinyl acetate. This insures thatthis ingredient effects a slight bonding which is believed to increasethe strength of the final article to some extent. When the articlereaches the exit end of the drying tunnel 64 it is ready to be assembledwith another such unit in inverted fashion to form a pocket section 12for eventual assembly in the replaceable filter unit 10.

Filtering units manufactured in the above described manner have provedhighly satisfactory in actual use. The strength of the filteringmaterial is more than adequate even for the large area of materialinvolved with pocket or bag filters of this nature. The airflowcapabilities are easily balanced against the dust-trapping capabilitiesof the filtering material, and these two requirements are determined bythe fiber types, diameters and lengths. For some applications, it isdesirable to have higher airflow capabilities and lower dust trappingcapabilities, and the ingredients of the fibrous material may becombined according to the above teachings to achieve the followingrequirements having desirable airflow and dust trapping capabilitiesthroughout a wide range.

Moreover, with filters made according to this invention, the air flowcapabilities are maintained at the desired high level even though thereplaceable unit experiences considerable clogging after long use. Thisis believed to be attributable to the fact that the downstream side 34of the article is the screen side of the article; that is, the side ofthe article which is in contact with the forming die 42 during themolding operation.

The upstream side 32 of the article is thus "fuzzier" and less compactdue to the manner in which the water is drawn through the article duringthe molding operation. When placed in use as a filter, the upstream sideof the article is thus fuzzier than the downstream side, and thismaterially improves the anticlogging characteristics of the filteraccording to this invention.

Since the forming die 44 includes a series of parallel slots beneath thescreen forming member, the final article includes parallel strips offibrous material which are more dense in the locations which were abovethe slots during the molding operation and less dense in the areas whichwere above the lands of the molding die. In the finished article, thedense ribs increase the strength of the article and the less dense ribswhich alternate therewith provide increased airflow capabilities. Withthe filtering unit of the area mentioned above, the porosity andstrength values are balanced so that airflow of approximately 10,000cubic feet per minute and higher are possible. At this flow rate, togive but one illustrative example, standard test dust comprised ofapproximately percent by weight carbon black and 3 percent by weightcotton linters and 27 percent by weight Arizona road dust is filteredfrom an air stream in amounts of 10 grams per 2 square feet at 0.08-0.16inches of H 0 pressure drop and at 67 cubic feet per minute of roomtemperature air.

While the above described embodiments constitute preferred modes ofpracticing this invention, other embodiments and equivalents may beresorted to within the scope of the actual invention, which is claimedas:

l. A method of molding a subassembly for incorporation in a replaceableunit for filtering contaminants from streams of gas comprising the stepsof a. preparing an aqueous slurry containing up to 0.4 percent by weightof a mixture of fibrous material having no substantial amount of phenolthermoset resin effective as a binder and including i. up to 30.0percent by weight ofwood pulp,

ii. 20-65 percent by weight of at least one member of the groupconsisting of olefin, acrylic and modacrylic fibers and fibers of acopolymer of vinylchloride and vinyl acetate, and

iii. a balance comprising fibers of rayon, a polyester, 21 polyamide,fiberglass or asbestos or a mixture of any two or more thereof,

b. immersing an open-face forming die contoured to define the filterunit subassembly in the aqueous slurry and c. applying suction throughthe fonning die to deposit a layer of the fibrous material thereon,

d. removing the forming die from the aqueous slurry with the layer offibrous material thereon,

e. pressing the layer against the contoured forming die by means of amatingly contoured transfer die and applying suction through thetransfer die to remove the layer of fibrous material from the formingdie,

g. releasing the layer from the transfer die, and

h. drying the layer to attain a molded filter unit subassembly.

2. The method of molding a filter unit subassembly as in claim 1 whereinthe mixture of fibrous material in the slurry comprises 2-30 percent byweight of wood pulp, 25-50 percent by weight of fibers of a copolymer ofvinyl chloride and vinyl acetate, l0-40 percent by weight of fibers ofrayon and l0-40 percent by weight of fibers of polyethyleneteraphthalate.

3. The method of molding a filter unit subassembly as in claim 2 whereinthe mixture of fibrous material is approximately 0.05 percent of theweight of the aqueous slurry, and the mixture of fibrous materialincludes about 20 percent by weight of wood pulp, about 30 percent byweight of fibers of a copolymer of vinyl chloride and vinyl acetate,about 30 percent by weight of fibers of rayon, and about 20 percent byweight of fibers of polyethylene teraphthalate, and the material hasadded thereto a flame retardant.

4. The method of molding a filter unit subassembly as in claim 2 whereinthe mixture of fibrous material is less than approximately 0.10 percentof the weight of the aqueous slurry, and the mixture of fibrous materialincludes about l percent by weight of wood pulp, about 35 percent byweight of fibers of a copolymer of vinyl chloride and vinyl acetate,about 40 percent by weight of fibers of fiberglass and about 15 percentby weight of fibers of polyethylene teraphthalate, and the material hasadded thereto a flame retardant.

5. The method of molding a filter unit subassembly as in claim 2 whereinthe mixture of fibrous material includes as wood pulp bleached softwoodkraft, the majority of the fibers of the copolymer of vinyl chloride andvinyl acetate have a length of approximately one-quarter inch and adiameter of approximately 15-20 microns, the fibers of rayon have alength of up to approximately three-eighths inch and a minor portionthereof have a diameter of approximately 15-20 microns and a majorportion thereof have a diameter of approximately 27-32 microns, and themajority of the fibers of polyethylene teraphthalate have a length ofapproximately three-eighths inch and a diameter of approximately 15-20microns.

6. The method of molding a filter unit subassembly as in claim 1 whereinthe aqueous slurry is prepared by first blending approximately 2 percentby weight of the mixture of fibrous material with approximately 98percent by weight water, and thereafter adding water to the blend toattain the desired aqueous slurry containing up to 0.4 percent by weightof fibrous material.

7, The method of molding a filter unit subassembly as in claim 1 whereinthe contoured open-face forming die comprises a slotted support membercovered with screen on which the layer of fibrous material is deposited,and suction is applied through rearward openings connecting with thebase of the slots, and the matingly contoured transfer die comprises aslotted member wherein the lands between the slots make direct contactwith the layer of fibrous material.

8. The method of molding a filter unit subassembly as in claim 7 whereinthe open-face forming die has approximately 80 percent of its exposedforming area consisting of screensupporting lands between the slots, andthe matingly contoured transfer die has approximately 95 percent of itsexposed article-contacting area composed of lands between the slots.

9. The method of molding a filter unit subassembly as in claim 7 whereinthe open-face forming die is immersed in the aqueous slurry and suctionat a value of approximately to 20 inches Hg is applied for approximately1 to l0 seconds, and the contoured fonning die thereafter lightlypresses the layer of fibrous material against the forming die forapproximately I second at room temperature prior to removing the layerfrom the forming die.

10. The method of molding a filter unit subassembly as in claim 2wherein the layer of fibrous material is free dried on a substantiallyplanar surface without constricting dies at a temperature in thesoftening range but below the melting point of the copolymer of vinylchloride and vinyl acetate.

11. An aqueous slurry having a solids content of up to 0.4 percent byweight having no substantial amount of phenol thermoset resin effectiveas a binder and comprising up to 30 percent by weight of wood pulp,20-65 percent by weight of at least one member of the group consistingof olefin, acrylic and modacrylic fibers and fibers of a copolymer ofvinyl chloride and vinyl acetate, and a balance comprising at least onemember of the group consisting of rayon fibers, polyethyleneteraphthalate fibers, fiberglass and asbestos fibers.

12. A suction molded fibrous mat having no substantial amount of phenolthennoset resin effective as a binder and comprising an intimateadmixture of up to 30 percent by weight of wood pulp, 20-65 percent byweight of at least one member of the group consisting of olefin, acrylicand modacrylic fibers and fibers of a copolymer of vinyl chloride andvinyl acetate, and a balance comprising at least one member of the groupconsisting of rayon fibers, polyethylene teraphthalate fibers,fiberglass and asbestos fibers, the fibers of said mat having an averagediameter smaller than about l00 microns and an average length of fromabout one-eighth inch to about one-half inch.

13. The suction molded fibrous mat as in claim 12 wherein the balanceincludes rayon fibers and polyethylene teraphthalate fibers.

14. The suction molded fibrous mat as in claim 13 wherein the intimateadmixture includes about 20 percent by weight of wood pulp, about 30percent by weight of fibers of a copolymer of vinyl chloride and vinylacetate, about 30 percent by weight of fibers of rayon, and about 20percent by weight of fibers of polyethylene teraphthalate.

15. The suction molded fibrous mat as in claim 12 wherein the intimateadmixture includes about 10 percent by weight of wood pulp, about 35percent by weight of fibers of a copolymer of vinyl chloride and vinylacetate, about 40 percent by weight of fibers of fiberglass, and about15 percent by weight of fibers of polyethylene teraphthalate.

16. The suction molded fibrous mat as in claim 12 wherein the wood pulpis bleached softwood kraft, the majority of the fibers of the copolymerof vinyl chloride and vinyl acetate have a length of approximatelyone-quarter inch and a diameter of approximately l5-20 microns, thefibers of rayon have a length of up to approximately three-eighths inchand a minor portion thereof have a diameter of approximately 15-20microns and a major portion thereof have a diameter of approximately27-32 microns, and the majority of the fibers of polyethyleneteraphthalate have a length of approximately three-eighths inch and adiameter of approximately 15-20 microns.

2. The method of molding a filter unit subassembly as in claim 1 whereinthe mixture of fibrous material in the slurry comprises 2-30 percent byweight of wood pulp, 25-50 percent by weight of fibers of a copolymer ofvinyl chloride and vinyl acetate, 10-40 percent by weight of fibers ofrayon and 10-40 percent by weight of fibers of polyethyleneteraphthalate.
 3. The method of molding a filter unit subassembly as inclaim 2 wherein the mixture of fibrous material is approximately 0.05percent of the weight of the aqueous slurry, and the mixture of fibrousmaterial includes about 20 percent by weight of wood pulp, about 30percent by weight of fibers of a copolymer of vinyl chloride and vinylacetate, about 30 percent by weight of fibers of rayon, and about 20percent by weight of fibers of polyethylene teraphthalate, and thematerial has added thereto a flame retardant.
 4. The method of molding afilter unit subassembly as in claim 2 wherein the mixture of fibrousmaterial is less than approximately 0.10 percent of the weight of theaqueous slurry, and the mixture of fibrous material includes about 10percent by weight of wood pulp, about 35 percent by weight of fibers ofa copolymer of vinyl chloride and vinyl acetate, about 40 percent byweight of fibers of fiberglas, and about 15 percent by weight of fibersof polyethylene teraphthalate, and the material has added thereto aflame retardant.
 5. The method of molding a filter unit subassembly asin claim 2 wherein the mixture of fibrous material includes as wood pulpbleached softwood kraft, the majority of the fibers of the copolymer ofvinyl chloride and vinyl acetate have a length of approximatelyone-fourth inch and a diameter of approximately 15-20 microns, thefibers of rayon have a length of up to approximately three-eighths inchand a minor portion thereof have a diameter of approximately 15-20microns and a major portion thereof have a diameter of approximately27-32 microns, and the majority of the fibers of polyethyleneteraphthalate have a length of approximately three-eighths inch and adiameter of approximately 15-20 microns.
 6. The method of molding afilter unit subassembly as in claim 1 wherein the aqueous slurry isprepared by first blending approximately 2 percent by weight of themixture of fibrous material with approximately 98 percent by weightwater, and thereafter adding water to the blend to attain the desiredaqueous slurry containing up to 0.4 percent by weight of fibrousmaterial.
 7. The method of molding a filter unit subassembly as in claim1 wherein the contoured open-face forming die comprises a slottedsupport member covered with screen on which the layer of fibrousmaterial is deposited, and suction is applied through rearward openingsconnecting with the base of the slots, and the matingly contouredtransfer die comprises a slotted member wherein the lands between theslots make direct contact with the layer of fibrous material.
 8. Themethod of molding a filter unit subassembly as in claim 7 wherein theopen-face forming die has approximately 80 percent of its exposedforming area consisting of screen-supporting lands between the slots,and the matingly contoured transfer die has approximately 95 percent ofits exposed article-contacting area composed of lands between the slots.9. The method of molding a filter unit subassembly as in claim 7 whereinthe open-face forming die is immersed in the aqueous slurry and suctionat a value of approximately 5 to 20 inches Hg is applied forapproximately 1 to 10 seconds, and the contoured forming die thereafterlightly presses the layer of fibrous material against the forming diefor approximately 1 second at room temperature prior to removing thelayer from the forming die.
 10. The method of molding a filter unitsubassembly as in claim 2 wherein the layer of fibrous material is freedried on a substantially planar surface without constricting dies at atemperature in the softening range but below the melting point of thecopolymer of vinyl chloride and vinyl acetate.
 11. An aqueous slurryhaving a solids content of up to 0.4 percent by weight having nosubstantial amount of phenol thermoset resin effective as a binder andcomprising up to 30 percent by weight of wood pulp, 20-65 percent byweight of at least one member of the group consisting of olefin, acrylicand modacrylic fibers and fibers of a copolymer of vinyl chloride andvinyl acetate, and a balance comprising at least one member of the groupconsisting of rayon fibers, polyethylene teraphthalate fibers,fiberglass and asbestos fibers.
 12. A suction molded fibrous mat havingno substantial amount of phenol thermoset resin effective as a binderand comprising an intimate admixture of up to 30 percent by weight ofwood pulp, 20-65 percent by weight of at least one member of the groupconsisting of olefin, acrylic and modacrylic fibers and fibers of acopolymer of vinyl chloride and vinyl acetate, and a balance comprisingat least one member of the group conSisting of rayon fibers,polyethylene teraphthalate fibers, fiberglass and asbestos fibers, thefibers of said mat having an average diameter smaller than about 100microns and an average length of from about one-eighth inch to aboutone-half inch.
 13. The suction molded fibrous mat as in claim 12 whereinthe balance includes rayon fibers and polyethylene teraphthalate fibers.14. The suction molded fibrous mat as in claim 13 wherein the intimateadmixture includes about 20 percent by weight of wood pulp, about 30percent by weight of fibers of a copolymer of vinyl chloride and vinylacetate, about 30 percent by weight of fibers of rayon, and about 20percent by weight of fibers of polyethylene teraphthalate.
 15. Thesuction molded fibrous mat as in claim 12 wherein the intimate admixtureincludes about 10 percent by weight of wood pulp, about 35 percent byweight of fibers of a copolymer of vinyl chloride and vinyl acetate,about 40 percent by weight of fibers of fiberglas, and about 15 percentby weight of fibers of polyethylene teraphthalate.
 16. The suctionmolded fibrous mat as in claim 12 wherein the wood pulp is bleachedsoftwood kraft, the majority of the fibers of the copolymer of vinylchloride and vinyl acetate have a length of approximately one-fourthinch and a diameter of approximately 15-20 microns, the fibers of rayonhave a length of up to approximately three-eighths inch and a minorportion thereof have a diameter of approximately 15-20 microns and amajor portion thereof have a diameter of approximately 27-32 microns,and the majority of the fibers of polyethylene teraphthalate have alength of approximately three-eighths inch and a diameter ofapproximately 15-20 microns.